Boundary Tension
and Wettability
Immiscible Phases
• Earlier discussions have considered only a
single fluid in the pores
– porosity
– permeability
• Saturation: fraction of pore space
occupied by a particular fluid (immiscible
phases)
–
Sw+So+Sg=1
• When more than a single phase is present,
the fluids interact with the rock, and with
each other
DEFINITION OF INTERFACIAL
TENSION
• Interfacial (boundary) tension is the energy
per unit area (force per unit distance) at the
surface between phases
• Commonly expressed in milliNewtons/meter (also, dynes/cm)
BOUNDARY (INTERFACIAL) TENSION
GAS

• Imbalanced molecular forces at phase boundaries
• Boundary contracts to minimize size
• Cohesive vs. adhesion forces
LIQUID
GAS
SOLID
Cohesive force
Adhesion force
Molecular
Interface
(imbalance
of forces)
LIQUID
(dense phase)
Modified from PETE
311 Notes
DEFINITION OF WETTABILITY
• Wettability is the tendency of one fluid
to spread on or adhere to a solid
surface in the presence of other
immiscible fluids.
• Wettability refers to interaction between
fluid and solid phases.
• Reservoir rocks (sandstone, limestone,
dolomite, etc.) are the solid surfaces
• Oil, water, and/or gas are the fluids
WHY STUDY WETTABILITY?
•Understand physical and chemical interactions between
• Individual fluids and reservoir rocks
• Different fluids with in a reservoir
• Individual fluids and reservoir rocks when multiple
fluids are present
•Petroleum reservoirs commonly have 2 – 3 fluids
(multiphase systems)
• When 2 or more fluids are present, there are at least 3
sets of forces acting on the fluids and affecting HC recovery
DEFINITION OF
ADHESION TENSION
• Adhesion tension is expressed as the
difference between two solid-fluid
interfacial tensions.
AT   os   ws    ow cos 
• A negative adhesion tension indicates that the denser phase (water)
preferentially wets the solid surface (and vice versa).
• An adhesion tension of “0” indicates that both phases have equal
affinity for the solid surface
CONTACT ANGLE
Oil
ow
Oil

os
Water
Oil
ws
os
Solid
AT = adhesion tension, milli-Newtons/m or dynes/cm)
The contact angle, , measured through
the denser liquid phase,
defines which fluid wets the solid
surface.
 = contact angle between the oil/water/solid interface measured through the water, degrees
os = interfacial energy between the oil and solid, milli-Newtons/m or dynes/cm
ws = interfacial energy between the water and solid, milli-Newtons/m or dynes/cm
ow = interfacial energy (interfacial tension) between the oil and water, milli-Newtons/m or dynes/cm
WETTING PHASE FLUID
• Wetting phase fluid preferentially wets the
solid rock surface.
• Attractive forces between rock and fluid draw
the wetting phase into small pores.
• Wetting phase fluid often has low mobile.
• Attractive forces limit reduction in wetting
phase saturation to an irreducible value
(irreducible wetting phase saturation).
• Many hydrocarbon reservoirs are either totally
or partially water-wet.
NONWETTING PHASE FLUID
• Nonwetting phase does not preferentially
wet the solid rock surface
• Repulsive forces between rock and fluid
cause nonwetting phase to occupy largest
pores
• Nonwetting phase fluid is often the most
mobile fluid, especially at large
nonwetting phase saturations
• Natural gas is never the wetting phase in
hydrocarbon reservoirs
WATER-WET RESERVOIR ROCK
• Reservoir rock is water - wet if water preferentially
wets the rock surfaces
• The rock is water- wet under the following
conditions:
• ws > os
• AT < 0 (i.e., the adhesion tension is negative)
• 0 <  < 90
If  is close to 0, the rock is considered
to be “strongly water-wet”
WATER-WET ROCK
ow

os
Oil
Water
ws
Solid
os
• 0 <  < 90
•
Adhesive tension between water and the
rock surface exceeds that between oil and
the rock surface.
OIL-WET RESERVOIR ROCK
• Reservoir rock is oil-wet if oil preferentially
wets the rock surfaces.
• The rock is oil-wet under the following
conditions:
• os > ws
• AT > 0 (i.e., the adhesion tension is positive)
• 90 <  < 180
If  is close to 180, the rock is considered to
be “strongly oil-wet”
OIL-WET ROCK
ow
Water
Oil

os
ws
os Solid
• 90 <  < 180
• The adhesion tension between water and the
rock surface is less than that between oil and the
rock surface.
INTERFACIAL CONTACT ANGLES,
VARIOUS ORGANIC LIQUID IN
CONTACT WITH SILICA AND CALCITE
WATER
SILICA
SURFACE
WATER
CALCITE
SURFACE
From Amyx Bass and Whiting, 1960; modified from Benner and Bartel, 1941
GENERALLY,
• Silicate minerals have acidic surfaces
• Repel acidic fluids such as major polar
organic compounds present in some crude oils
• Attract basic compounds
• Neutral to oil-wet surfaces
• Carbonate minerals have basic surfaces
• Attract acidic compounds of crude oils
• Neutral to oil-wet surfaces
Tiab and Donaldson, 1996
Caution: these are very general statements and relations
that are debated and disputed by petrophysicists.
WATER-WET
OIL-WET
Air
OIL




WATER
 < 90
SOLID (ROCK)
FREE WATER
OIL
Oil
WATER
WATER
WATER
 > 90
SOLID (ROCK)
OIL
GRAIN
GRAIN
OIL
RIM
BOUND WATER
FREE WATER
Ayers, 2001
WATER-WET
OIL-WET
Oil
Air


WATER
WATER
From Levorsen, 1967
Brown, G.E., 2001, Science, v. 294, p. 67-69
n = 30 silicate and 25 carbonates
From Tiab and Donaldson, 1996
CONTACT ANGLE: Triber et al.
-Water-wet = 0 – 75 degrees
-Intermediate-wet = 75 – 105 degrees
-Oil-wet = 105 – 180 degrees
n = 161 ls., dol.
CONTACT ANGLE:
-Water-wet = 0 – 80 degrees
-Intermediate-wet = 80 – 100 degrees
-Oil-wet = 100 – 180 degrees
WETTABILITY IS AFFECTED BY:
• Composition of pore-lining minerals
• Composition of the fluids
• Saturation history
WETTABILITY CLASSIFICATION
• Strongly oil- or water-wetting
• Neutral wettability – no preferential wettability
to either water or oil in the pores
• Fractional wettability – reservoir that has local
areas that are strongly oil-wet, whereas most
of the reservoir is strongly water-wet
- Occurs where reservoir rock have variable
mineral composition and surface chemistry
• Mixed wettability – smaller pores area water-wet
are filled with water, whereas larger pores are
oil-wet and filled with oil
- Residual oil saturation is low
- Occurs where oil with polar organic compounds
invades a water-wet rock saturated with brine
IMBIBITION
• Imbibition is a fluid flow process in which
the saturation of the wetting phase
increases and the nonwetting phase
saturation decreases. (e.g., waterflood of an
oil reservoir that is water-wet).
• Mobility of wetting phase increases as
wetting phase saturation increases
– mobility is the fraction of total flow capacity for a particular
phase
WATER-WET RESERVOIR,
IMBIBITION
• Water will occupy the smallest pores
• Water will wet the circumference of most larger pores
• In pores having high oil saturation, oil rests on a water film
• Imbibition - If a water-wet rock saturated with oil is
placed in water, it will imbibe water into the smallest
pores, displacing oil
OIL-WET RESERVOIR,
IMBIBITION
• Oil will occupy the smallest pores
• Oil will wet the circumference of most larger pores
• In pores having high water saturation, water rests on an
oil film
• Imbibition - If an oil-wet rock saturated with water is
placed in oil, it will imbibe oil into the smallest
pores, displacing water
e.g., Oil-wet reservoir – accumulation of oil in trap
DRAINAGE
• Fluid flow process in which the
saturation of the nonwetting phase
increases
• Mobility of nonwetting fluid phase
increases as nonwetting phase
saturation increases
– e.g., waterflood of an oil reservoir that is oil-wet
– Gas injection in an oil- or water-wet reservoir
– Pressure maintenance or gas cycling by gas injection
in a retrograde condensate reservoir
– Water-wet reservoir – accumulation of oil or gas in trap
IMPLICATIONS OF WETTABILITY
• Primary oil recovery is affected by the
wettability of the system.
– A water-wet system will exhibit
greater primary oil recovery.
WATER-WET
OIL-WET
Air
OIL




WATER
 < 90
SOLID (ROCK)
FREE WATER
OIL
Oil
WATER
WATER
WATER
 > 90
SOLID (ROCK)
OIL
GRAIN
GRAIN
OIL
RIM
BOUND WATER
FREE WATER
Ayers, 2001
IMPLICATIONS OF WETTABILITY
• Oil recovery under waterflooding is
affected by the wettability of the
system.
– A water-wet system will exhibit
greater oil recovery under
waterflooding.
Water-Wet System
Oil-Wet System
Effect on waterflood of an oil reservoir?
From Levorsen, 1967
IMPLICATIONS OF WETTABILITY
• Wettability affects the shape of the
relative permeability curves.
– Oil moves easier in water-wet rocks
than oil-wet rocks.
Recovery efficiency, percent, Soi
IMPLICATIONS OF WETTABILITY
Core Percent
no silicone Wettability
1
2
3
4
5
80
1
2
3
60
0.00
0.020
0.200
2.00
1.00
0.649
0.176
- 0.222
- 0.250
- 0.333
Curves cut off at Fwd •100
4
40
?
p. 274
5
20
0
1
2
3
4
5
6
7
8
9
10
11
12
Water injected, pore volumes
Modified from Tiab and Donaldson, 1996
Recovery efficiency, percent Spi
IMPLICATIONS OF WETTABILITY
Squirrel oil - 0.10 N NaCl - Torpedo core ( • 33 O W • 663,
K • 0945, Swi • 21.20%)
Squirrel oil - 0.10 N NaCl • Torpedo Sandstone core,
after remaining in oil for 84 days ( • 33.0 W • 663, K •
0.925, Swi • 23.28%)
80
60
40
20
0
1
2
3
4
5
6
7
8
Water injection, pore volumes
9
10
Modified from NExT, 1999
WETTABILITY AFFECTS:
• Capillary Pressure
• Irreducible water saturation
• Residual oil and water saturations
• Relative permeability
• Electrical properties
LABORATORY MEASUREMENT OF
WETTABILITY
Most common measurement techniques
– Contact angle measurement method
– Amott method
– United States Bureau of Mines
(USBM) Method
NOMENCLATURE
AT = adhesion tension, milli-Newtons/m or dynes/cm)
 = contact angle between the oil/water/solid interface measured through
the water (more dense phase), degrees
os = interfacial tension between the oil and solid, milli-Newtons/m or
dynes/cm
ws = interfacial tension between the water and solid, milli-Newtons/m or
dynes/cm
ow = interfacial tension between the oil and water, milli-Newtons/m or
dynes/cm
References
1. Amyx, J.W., Bass, D.M., and Whiting, R.L.: Petroleum Reservoir Engineering, McGrow-Hill Book
Company New York, 1960.
2. Tiab, D. and Donaldson, E.C.: Petrophysics, Gulf Publishing Company, Houston, TX. 1996.
3. Core Laboratories, Inc. “A course in the fundamentals of Core analysis, 1982.
4.
Donaldson, E.C., Thomas, R.D., and Lorenz, P.B.: “Wettability Determination and Its Effect
on Recovery Efficiency,” SPEJ (March 1969) 13-20.